Hydrogen fuel cells generally do not consume all the hydrogen or oxygen supplied to them, as this would allow liquid water and inert gases to accumulate in the areas of the cell that are close to the end of the gas channel, resulting in severe performance loss due to low reactant concentration. Instead, such fuel cells are generally provided with some excess gas. In small systems or research fuel cells, the excess gas may simply be vented for the purposes of simplicity. In a large system (e.g. like that used to power a vehicle), however, the requirements of fuel efficiency necessitate that the excess hydrogen gas be recirculated for later use.
Positive displacement mechanical pumps have generally been used for recirculating hydrogen in a fuel cell system; they are usually of the vane or Roots® blower type. Whatever their mechanical details, they all use some solid moving part to push the hydrogen through the system. The design and operation of such mechanical pumps or blowers, however, is a serious engineering challenge, because the moist hydrogen in fuel cell systems corrodes many common materials (impervious materials are generally limited and/or costly), and the moving parts cannot be lubricated with any oil or grease that might contaminate the fuel cell stack. In addition, such devices require substantial energy to operate, and thus increase the balance-of-plant load, decreasing the overall efficiency of the fuel cell system.
One alternative to the above design includes the use of mechanical diaphragms. Systems that require less pressure differential from fuel cell inlet to outlet, and consequently lower hydrogen supply pressure, may use a dual mechanical diaphragm. Such a system is disclosed in U.S. Pat. No. 6,858,340. The pressure differential between hydrogen and air in the arrangement described in the above-referenced patent, however, generally depends on hydrogen supply pressure and decreases with increasing absolute air pressure.
Another alternative includes the use of fixed geometry ejectors. US Patent Application Publication No. 2008/0118371 refers to an ejector that has many similarities to manually adjustable steam ejectors well-known in engineering practice since at least the early 20th century. The ejector described in the above-described application, however, may only perform adequately or optimally at a specific gas flow rate or within a relatively small range of flow rates. Accordingly, improvements in systems and devices for recirculating hydrogen in fuel cells are desired.